From EP 0 401 600 such a device is already known for a test material in thread or wire form, wherein provision is made for a measuring gap, on each of the side walls of which a measuring electrode forming part of a capacitive measuring element is provided. In addition to the capacitive measuring element an optical measuring element is provided, comprising a light source arranged on one side of the measuring gap and a photoelectric element arranged on the other side of the measuring gap. With a view to generating a homogeneous field of illumination in the measuring gap, an aperture or a light guide in the form of a truncated cone is provided between the light source and the measuring gap.
A disadvantage of this known device can be seen in the fact that, in addition to the measuring gap, a lot of space is taken up for the light source and for means for guiding the beam of light. In addition, these means and the light source may have to be separably attached to one another or to a support stand, resulting overall in a solution that saves little space and is not very cost-effective.